Electric power transformers are commonly filled with an oil which serves as a dielectric and as a heat transfer medium. Such oils must be very stable, i.e. chemically inert, in order that physical and electric properties of the oil shall not change in service. They must also be capable of free flow at low temperatures to perform the heat exchange function and also to disperse any degradation products which may arise from corona discharge within the transformer. For like reasons, the oil must be of low or moderate viscosity. Flash and fire points are also required properties in order that a temporary rise in temperature of the equipment shall not create an undue risk of fire.
High flash and fire points are achieved by employing petroleum fractions of high boiling point. But, in general, higher boiling point cuts are of higher viscosity. The compromise to achieve acceptable flash and fire points and acceptable viscosity results in selection of fractions within the boiling range of about 450.degree.-1050.degree. F., the range in which are found the straight and slightly branched paraffins which solidify at temperatures such as to cause the total fraction to fail the cloud point and pour point test specifications for transformer oils.
For the reasons stated it has been the practice of the petroleum refining industry to prepare transformer oils from naphthenic base crude fractions of suitable boiling range. The cost of dewaxing other crudes to the low pour point required of transformer oil by the conventional solvent dewaxing equipment presently available in refineries is so high as to be impracticable. Thus refiners have met a -30.degree. F. or lower pour point specification by treatment of naphthenic distillates to such an extent that the term "transformer oil" has been acceptable as meaning refined from a naphthenic distillate. Remarks similar to those just made about transformer oils apply equally well to refrigerator oils, i.e. oils used to lubricate refrigeration compressors.
In recent years techniques have become available for catalytic dewaxing of petroleum stocks. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal dated Jan. 6, 1975, at pages 69-73. See also U.S. Pat. No. 3,668,113.
In U.S. Pat. No. Re. 28,398 is described a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such process combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938.